The biliary excretion and enterohepatic circulation of two polychlorinated hydrocarbons, 4-chlorobiphenyl and 2,3,6,2',3',6'-hexachlorobiphenyl, have been studied in the rat. Both PCBs show similar biliary excretion and absorption from the intestine yet the ultimate route of elimination differs between the two compounds being more urinary than biliary in the case of 4-chlorobiphenyl but entirely biliary for the hexachlorobiphenyl. The rate of elimination of the hydryoxylated metabolites of 4-chloro- and 2,2',5,5'-pentachlorobiphenyl is more rapid than for the parent compounds especially in the latter case. Again, the ultimate fate of the two compounds differs despite an initially similar extent of biliary elimination. 3'-Hydroxy-2,2'4,5,5'-pentachlorbiphenyl is excreted entirely into the feces, whereas 4-hydroxy-4'-chlorobiphenyl is eliminated equally in the urine and the feces.